The overall goal of this proposal is to address a widespread clinical problem occurring after the Fontan operation, persistent pleural effusions, through the investigation of vascular endothelial barrier function. The foundational hypothesis of this proposal is that children with single ventricle physiology who undergo the Fontan operation experience derangements in vascular permeability secondary to the profound inflammatory effects of cardiopulmonary bypass (CPB). Therefore, identifying therapeutic targets that restore vascular endothelial barrier may be critical to improving post-operative care after the Fontan operation. Sphingosine 1-phosphate (S1P) is a phospholipid that interacts with G protein coupled receptors on endothelial cells and mediates enhancement of endothelial barrier function, thus reducing vascular permeability. The critical role of S1P in regulating vascular permeability in vivo has been illustrated in animal models. For example, mice deficient in one of the two enzymes responsible for S1P biosynthesis, sphingosine kinase-1, display increased pulmonary microvessel permeability and greater levels of edema formation in response to an inflammatory insult induced with lipopolysaccharide (LPS) or PAR-1 activation. Importantly, intravenous administration of S1P in mice and dogs has been shown to play a protective role in acute lung injury by reducing pulmonary vascular leakage and intrapulmonary shunting. Our preliminary analysis of plasma from Fontan patients has revealed a reduction in postoperative plasma levels of HDL, the principle carrier of the endothelial barrier- promoting sphingolipid, sphingosine-1-phosphate (S1P). Based on these findings we hypothesize that plasma HDL-S1P levels are lowered in children after exposure to CPB and are predictive of persistent pleural effusions and longer post-operative recovery time. Additionally, we hypothesize that blood samples from children with persistent pleural effusions will display evidence of either propensity to disrupt barrier or a lack of ability to promote barrier either of which can be overcome by exogenous administration of S1P. To address these inter-related hypotheses there are two specific aims: 1) to determine whether plasma levels of S1P correlate with clinical markers of prolonged postoperative recovery (i.e., persistent pleural effusions) after the Fontan operation, and 2) to determine the level of barrier integrity induced by plasma samples from Fontan operation patients and determine if this correlates with S1P levels and/or severity of post-operative pleural effusions. These later studies will also determine whether exogenous administration of S1P can increase the capacity of the samples to enhance barrier in vitro to levels achieved using preoperative samples. The aims of the application may lead to new S1P based strategies for treatment of derangements in vascular permeability in Fontan patients and other infants and children undergoing CPB.